Recently demand for electric and electronic apparatuses which are small-sized and portable, such as audio tape recorder, camera built-up video tape recorder, personal computer and portable phone has been increasing more and more. As a result of such an increased demand, high performance secondary battery which is small-sized, light and rechargeable has come to be required. In addition to conventional lead storage batteries and nickel-cadmium batteries, various new batteries of nickel-metal hydride type and lithium ion type have been commercialized. Among them, nickel-metal hydride secondary batteries employing alkaline electrolyte have problems remaining unsolved, that is to say, a voltage is low, energy density cannot be increased and a self-discharge is large. On the other hand, lithium ion secondary batteries employing non-aqueous electrolyte have merits such as a high voltage, high energy density, small self-discharge and extra-light weight, and are expected to be developed greatly in the future.
A point for enhancing energy density of the lithium ion secondary batteries is a technique for producing an electrode thereof. In case where a negative electrode is produced by using a carbonaceous material such as coke, carbon or the like as an active material for negative electrode, first the carbonaceous material is powdered, and then dispersed in a solvent together with a binder to prepare a negative electrode composition. After the composition is coated on a current collector for negative electrode, the solvent is removed by drying and the coated current collector is rolled, thus giving the negative electrode. Hereinafter a carbonaceous material merely storing and releasing a lithium ion is also called an active material. Similarly a positive electrode is produced, for example, by powdering a lithium-containing oxide as an active material for positive electrode, dispersing the powder in a solvent together with a conductive agent and binder to prepare a positive electrode composition, coating the composition on a current collector for positive electrode, removing the solvent by drying and then rolling the coated current collector. As a binder for lithium ion secondary batteries, polyvinylidene fluoride has been used widely. For example, JP-A-4-249859 discloses a technique for producing electrode sheets, in which a lithium-containing oxide such as LiCoO.sub.2 as an active material for positive electrode and graphite as a conductive agent are mixed with polyvinylidene fluoride to prepare a positive electrode composition, the obtained composition is dispersed in N-methylpyrrolidone to give a slurry, the slurry is applied onto an aluminum foil current collector for positive electrode and then the coated current collector is dried and compression-molded with a roller press, thus giving a positive electrode sheet, and separately in which a carbonaceous material as an active material for negative electrode is mixed with polyvinylidene fluoride to prepare a negative electrode composition, the obtained composition is dispersed in N-methylpyrrolidone to give a slurry, the slurry is coated on a copper foil current collector for negative electrode and then the coated current collector is dried and compression-molded with a roller press, thus giving a negative electrode sheet. However, a solvent of polyvinylidene fluoride is limited to expensive specific organic solvents having a high boiling point, such as N-methylpyrrolidone, dimethylformamide and dimethylacetamide. Therefore it takes a lot of time to dry the solvent at the time of producing an electrode sheet and production cost increases. Also polyvinylidene fluoride is apt to swell in an organic solvent used for a non-aqueous electrolyte of lithium ion secondary battery, such as propylene carbonate, ethylene carbonate, diethyl carbonate or a mixture thereof. For that reason, there arise problems that as charging and discharging are repeated, adhesion to a metal foil as a current collector becomes poor and as a result, increase in internal resistance of a battery occurs and battery performance is lowered. Further an electrode sheet produced by using a polyvinylidene fluoride binder is poor in flexibility, and when the electrode sheet is folded by 180 degrees for producing a square form battery or when wound for producing a small cylindrical form battery, there easily occur a problem that an electrode composition is separated from the electrode sheet, which results in decrease in yield. Also JP-A-4-95363 discloses the use, as a binder, of a material having rubber elasticity and comprising mainly a fluorine-containing copolymer such as vinylidene fluoride-hexafluoropropylene copolymer or vinylidene fluoride-chlorotrifluoroethylene copolymer for the purpose to enhance a binding property against expansion and shrinkage of an active material for positive electrode at the time of charging and discharging of a secondary battery using a non-aqueous electrolyte. However since those copolymers have crystallinity lower than that of polyvinylidene fluoride, they are apt to swell against an organic solvent of a non-aqueous electrolyte as compared with polyvinylidene fluoride and elution thereof occurs depending on kind of a non-aqueous electrolyte. Thus they do not function as a binder. JP-B-8-4007 discloses the use of similar binder such as fluorine-containing high molecular copolymer mainly comprising vinylidene fluoride, tetrafluoroethylene and hexafluoropropylene instead of polyvinylidene fluoride. Claims of that patent publication discloses a copolymer comprising 0.3 to 0.9% by mole of vinylidene fluoride, 0.03 to 0.5% by mole of hexafluoropropylene and 0 to 0.5% by mole of tetrafluoroethylene, in which a total % by mole of those three monomers is from 0.80 to 1. In that patent publication, too, it is pointed out that since polyvinylidene fluoride is soluble only in the above-mentioned specific solvents such as N-methylpyrrolidone, dimethylacetamide, dimethylformamide and methyl sulfoxide which have a high polarity and a high boiling point and some of which are toxic, when an electrode is produced by coating an active material by using the mentioned solvent and then molding, there are problems, from the viewpoint of production process, that it takes too long period of time to dry the solvent having a high boiling point and due to toxicity of the solvent, sealing equipment and exhaust equipment are required. In that patent publication, in order to solve the above-mentioned problems, the mentioned copolymer which dissolves in usual organic solvents costing low and having a low boiling point, for example, ketone solvents such as methyl ethyl ketone and methyl isobutyl ketone, ester solvents such as ethyl acetate and butyl acetate, ether solvents such as dioxane and tetrahydrofuran and a mixture thereof is used for a binder. However, since a degree of swelling of that copolymer is basically large against an organic solvent of non-aqueous electrolyte like the above-mentioned vinylidene fluoride-hexafluoropropylene copolymer and vinylidene fluoride-chlorotrifluoroethylene copolymer, during a long term use of a battery, peeling of a battery electrode composition from a current collector and releasing of an active material occur, which arises a problem with lowering of battery performance.
Further JP-A-7-147156 describes that by using an electrode produced by adhering a composite layer of an insoluble non-melting substrate having a polyacene structure and a specific binder to a metal foil, battery performance is enhanced and, that the binder is a fluorine-containing polymer having a fluorine atom/carbon atom ratio of less than 1.5 and not less than 0.75. However a polymer disclosed in that patent publication is only polyvinylidene fluoride. For example, both ethylene-tetrafluoroethylene copolymer and propylene-tetrafluoroethylene copolymer disclosed therein are insoluble in an organic solvent, and seem not suitable as a binder and not practical from the description in the publication that in order to obtain a uniform electrode, it is preferable that a fluorine-containing polymer is completely dissolved.
An object of the present invention is to provide a binder for secondary battery using non-aqueous electrolyte which has flexibility as compared with polyvinylidene fluoride, is soluble in not only conventional solvents such as N-methylpyrolidone, dimethylacetamide and dimethylformamide but also an organic solvent having a low boiling point such as acetone or methyl ethyl ketone and, as compared with the above-mentioned fluorine-containing copolymer, is less swelling against organic solvents of non-aqueous electrolyte such as propylene carbonate, ethylene carbonate, diethyl carbonate, diethoxyethane and a mixture thereof, and to provide a battery electrode composition which is prepared by using the binder.